In producing integrated circuits by photoetching, a glass mask on which a pattern corresponding to an integrated circuit has been formed using a metal, such as chromium, is placed on a photoresist layer coated on a silicon substrate, and is then exposed to light to form a latent image of the pattern on the photoresist layer. After the exposure, processings such as development and etching are generally conducted to obtain the desired integrated circuit.
Such glass masks for use in photoetching must have a small coefficient of thermal expansion and be free of air bubbles or striaes. Further, such glass masks must meet the following requirements:
(1) They permit sharp patterns to be formed thereon which do not have defects such as pinholes in a chromium film.
(2) They have high adhesion to the chromium film and are resistant to a heat treatments or supersonic wave cleaning.
(3) They are resistant to strong acids and/or alkalis which are used in, for example, washing treatments and peeling apart photoresist layers.
Conventional glass compositions for photoetching masks include SiO.sub.2 --B.sub.2 O.sub.3 -alkali metal oxide and SiO.sub.2 --Al.sub.2 O.sub.3 --alkaline earth metal oxide compositions.
The SiO.sub.2 --B.sub.2 O.sub.3 -alkali metal oxide compositions are relatively stable glass compositions. However, since a large amount of boric acid is used as a starting material, striaes tend to be formed due to volatilization during melting of the glass compositions and this makes it difficult to obtain a homogeneous glass.
On the other hand, the SiO.sub.2 --Al.sub.2 O.sub.3 -alkaline earth metal oxide compositions have the advantage that the glasses obtained therefrom have a small coefficient of thermal expansion, but have the defect that the glasses are unstable and tend to devitrify and also the viscosity of the glass compositions during melting is high.